We don't do it often, as we do LOVE our colour, but sometimes you can go black and white. We find we use black and white more when we are having trouble correcting colour or if the image is over or under exposed. Black and white can be a little more forgiving, making the grain / noise a lot more subtle.
As you can see in this pic, the horses bottom are a little blown out (oops! poor horsey). But by making it black and white and pulling back the highlights this photo has been saved. Hi five Peta!
Kate turned this image black and white because he kitchen has a less than desirable colour scheme and tiles. She really just wanted you to focus on the steam from the percolator, not the fake terracotta and country theme border tiles.
This is a great example of how black and white really makes you focus on the emotion of the image and nothing else. If this image had been in colour you would have been distracted by the colours and patterns of the clothing instead of focussing on the cute and happy family. These are one of our fave kind of black and whites.
There is another cool thing about black and white. It adds a real timeless quality to your images. This one was taken on our road trip, see how this could be anytime. I guess it does help that it was taken in an awesome car! A tip for when you are photographing your kids (and other people you can tell what to wear) is to dress them in unlogo-ed and unlicensed clothing. It will date and cheapen you photo in a flash!
So, black and white can show off graphic elements, emotions and cover up ugly colour schemes. Look out for a scene that screams "black and white" and shoot it!